Let's start with a simple truth," the foreign secretary, David Milliband, urged delegates at the Labour party conference in Brighton last week. "The earth does revolve around the sun – just not the one that's printed in Wapping." His words did not stop many of those gathered on the south coast from going into orbit when it first emerged that the paper planned to run a front-page editorial declaring "Labour has lost it".

In an era when consumers get their news from a greater variety of sources, the Sun may not wield the power and influence it once did. Yet the timing of the announcement, which overshadowed Gordon Brown's final conference speech before a general election, demonstrates that the paper's trademark chutzpah is undiminished, despite flagging sales. It may also have done more than anything to start the election campaign.

The Sun made its irritation with New Labour clear as long ago as the 2005 election, when it criticised Tony Blair's domestic policies. Executives told Brown months ago it was unlikely to support him if he didn't hold a referendum on the Lisbon treaty and the paper also came out for the Tories at the European elections.

Final decision

So it is little wonder that when News International chief executive Rebekah Brooks informed Peter Mandelson by phone of the Sun's landmark decision, he said Brown "would not be surprised" by it.

It is clear that the paper's "dossier on their failures" was preplanned. Within hours of publication, posters and vans featuring the paper's "We're feeling Blue" tagline could be spotted all over Brighton. Even so, Sun insiders claim that they did not take the final decision to publish until after Brown's conference speech on Tuesday. "We did wait for the speech," insists one, "but it would have had to be the speech of his life." Brown's address did contain some proposals – including a crackdown on antisocial behaviour – that might have raised a cheer in the Sun newsroom, or at the Daily Mail, but it did not impress NI executives, who speak disparagingly about the lack of a killer line.

After learning about the paper's intentions, Brown was furious enough to snub the News International party that same night, which was hosted by Brooks in the Empress Room at Brighton's Grand Hotel. It was an apt choice of venue; Brooks, who edited the Sun for six years before starting her new job last month, is now installed as the Queen of Wapping. Her hand-picked successor Dominic Mohan has apparently not met Brown since his appointment and shares Brooks's reluctance to talk to the press: "Thanks, but I think I'll let the paper speak for itself," he said, when approached.

Anyone who knows how News International operates, however, laughs off the suggestion that News Corporation's chairman and chief executive Rupert Murdoch played no part in the decision to ditch Brown.

After initially dismissing David Cameron, Murdoch's animus has cooled, and the pair have dined together at Cameron's home. Despite that, Mandelson told friends last week that the decision to abandon Labour was made by James Murdoch, who was given control of News Corp's European and Asian operations earlier this year. If so, it signifies a dramatic shift in power from father to son.

What does the Sun's move mean for both major parties? At the very least, Cameron's speech at the Conservative party conference in Manchester this week will be pored over for evidence that he is talking the Sun's language, which could prompt a more effusive endorsement. It is inconceivable that James Murdoch's increasingly vocal campaign against the BBC has not been noted by the Tory leader, who has issued his own attacks on the licence fee and executive pay in recent months. Senior BBC executives claim they are relaxed about the sabre rattling emanating from the Murdoch empire. "There would be a public uproar if it looks like they [the Tories] are doing a deal with Murdoch and come into power and then dump over the BBC big time," says one.

Cameron has also targeted another Murdoch bete noire, declaring that Ofcom "will cease to exist" in its current form under a Tory government. Senior Sun insiders suggest the Tory leader will now be pressed into service by the paper. "The message for the Tories is 'you now you have to earn Sun readers' support'," says a senior NI executive. "By coming out for the Tories it puts them on notice. They have to give us policy after policy and story after story – it keeps their feet to the fire." Last week's editorial criticised Labour without praising Cameron. In 1997, in contrast, the paper boldly declared: "We're backing Blair".

With the so-called quality press likely to keep its powder dry until nearer the election, Brown can perhaps take some comfort from the fact that Cameron's policies may be subject to greater scrutiny. The Sun's stablemate, the Times, which supported Blair in 2001 and 2005, has made it clear it has a problem with Labour's leader, but it has yet to back Cameron. It will embark on a forensic examination of Tory policies before making a public statement much closer to the election, according to a senior editorial figure.

Senior NI sources deny any suggestion that Brown will be subject to the same vitriol that bedevilled Neil Kinnock's campaigning. That does not change the fact that Brown has all but given up on the media after relations reached a new nadir during his trip to the US last month. Newspaper revelations that several Downing Street requests for meetings with Barack Obama were turned down overshadowed his visit, and attempts to kill the story by persuading the White House to phone Nick Robinson, the BBC's political editor, backfired. "The order came down to ostracise the Telegraph, the Guardian [which led on the same story] and the BBC," says another political editor.

Brown is convinced he can no longer use the media as a vehicle for communicating his policies to the public, as his on-screen irritation during the party conference demonstrated. That conviction was strengthened by Andrew Marr's questions about his health last Sunday. Aides say plans to take his message directly to the public on a nationwide tour with a series of public meetings are well advanced.

Brown's new approach borrows from Blair's strategy of using daytime TV and local papers to circumnavigate the national media, but it also has echoes of John Major's 1992 election campaign, when he took to the streets with a megaphone and a soapbox. Like Major after 1992, Brown now faces almost universally hostile media.

When News International's party ended in the early hours of Wednesday, revellers – including the culture secretary, Ben Bradshaw – retired to the Grand's bar to sing songs around a piano. By the time they belted out the old ELO hit Mr Blue Sky, the Sun's presses were already rolling. The title already seemed remarkably prescient.